Worth it to balance rotating assembly?

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Craig Burriss

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Hey guys, I’m building a 440 for my 4x4 RamCharger. In the next couple of weeks I’ll be taking the block, heads, and rotating assembly to a machine shop. I’ll probably need 30 over pistons, but plan on using the factory forged crank and factory rods.
For those of you that have had your engines balanced, was it worth it?
I’m thinking since this engine won’t see much rpm it’s not going to make that much of a difference. It’ll probably never see anything over 5500rpm, if that.
 
Yes, you should balance the rotating assembly when replacing the pistons. Typically, after market pistons are significantly lighter than stock pistons. 65'
 
If you can balance the new pistons to the weight of the original you should be able to stop there, if they are lighter you need to do a full balance
 
This is assuming the crank and rods as an assembly were balanced in the first place. Just get it done, much more $$ do do it later if there was an imbalance problem.
5500 RPM's if imbalanced enough causes huge loads on the rods/bolts etc... Like not balancing your tires because you probably won't go over 70 mph.
 
For your application, I would just balance the new pistons. Find the lightest one, & bring the rest down to this weight. You can do this with kitchen scales. 50% is a popular balance factor, but you may end up slightly under [ less than 50% ] or slightly over. Even experienced builders do not agree on balance factor.
 
Think of the mayhem that can ensue if something isn't right.
 
It's always nice to have a tighter than stock balance job. I've also just bought replacement type pistons and run it.

440, speed pros, no problem.
Lighter on the piston... to a point..is better than heavier.
 
Yes neutral balance it, while you're at it have the rods rebuilt with new bolts & nuts. I also would have it main line honed with new bolts or studs.
Both will make the neutral balance last with no chance for irregular wear to begin.
Balancing only costs a few 100 bucks.
That's me spending your money, but I do it at work & see the difference.
Good luck
 
The factory balance job on these engines sucks. Unless I'm on a super-tight budget and using stock replacement pistons I make it a point to balance every engine I build. I'm currently building a 440 for my D200 which will see lots of towing and probably rarely spin over 5000 RPM but I got Icon forged pistons for it which weigh almost half as much as stock cast pistons along with 440Source forged factory replacement rods. I had the crankshaft balanced to the new piston/rod weights and there was a significant amount of metal they drilled out of the counterweights which is all good; faster revving, less parasitic power loss, less stress and wear on bearings.

BTW the very first car engines didn't have counterweighted crankshafts; they were added later (first bolted on then cast/forged integrally) primarily to reduce loads on the main bearings, the reduced vibration was a secondary benefit. Even 100 years ago with engines that could barely turn 3000 RPM they knew keeping the rotating assembly balanced was very important.

Here's a question, sorry if I'm hijacking... if my crank was already balanced to the rods and pistons would it be worthwhile to equalize the piston and rod weights themselves after the fact? I might call back my machine shop and see what they recommend, I imagine they just picked one of the pistons/rods and used that for the bob weights? I'm not familiar with standard procedures for balancing a crankshaft in that regard. And I imagine the weights of these aftermarket parts have a lot less variation than 50-year-old OE stuff.

EDIT: Just to throw out there too, my machine shop charged $215 for balancing the crank... not sure if that's consistent with most machine shops since I kinda-sorta know the owner on a personal level but at that price I think it's very worthwhile. If it was closer to $500 then I might say save your money.
 
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My guy weighs each piston with rings and pin and the retainers if they are part of the assambly. Then each big end of the rod then each small end of the rod equalizing the weights of each end. Also included are the rod bearings. Then he weighs the crank and figures the bob weight. Kim
 
Factory balance is a joke, there is no way I would build anything without balancing.
Any out of balance weight squares with evert 2k RPM of speed.
 
For your application, I would just balance the new pistons. Find the lightest one, & bring the rest down to this weight. You can do this with kitchen scales. 50% is a popular balance factor, but you may end up slightly under [ less than 50% ] or slightly over. Even experienced builders do not agree on balance factor.
Forgive me for probably the dumb question, but what do you mean by 50% balance factor?
 
I don't think you will feel much of a difference or any HP gains worth noting. What you will benefit from is less engine wear, especially at higher rpm's.
 
Forgive me for probably the dumb question, but what do you mean by 50% balance factor?


50% of the reciprocating weight is what is used in figuring bob weight. 100% of the rotating weight plus 50% of reciprocating weight is the bob weight
 
Always have them balanced, every time rotating assembly parts are changed. When the rear view mirror walks around on its ball at 50 mph, and your vision gets fuzzy as the dash screws turn in and out by themselves at 60 mph, and the rear main seal leaks after only 20 miles you will kick yourself for being stingy. As at that point you face a complete tear down and reassembly which is lots of time, gaskets, and two more round trips to the machine shop. Just do it right the first time and sleep well.
 
Can of worms time here lol.

Never seen a factory balance anywhere near RIGHT!
I've done hundreds lol...
The only one I would say was even close was a 69 340 engine...

This is something I did for a living, good fun indeed.

Balancing the crank is in fact the easy part of a full balance!

Pistons are the hardest part to balance as they don't have many areas for lightening.

But when it comes to RODS you can 'have a ball' lol.

If your gonna spend out for a balance job, then give them rods a good seeing too!

Always deburr the beams and a light polish and grind them balance weights off each end by at least 10grams smallend and 20grams Bigend...

I can get a sixpack rod down to the weight of an LY rod!

Just imagine how much weight you can get off an LY lol...

Why bother you gonna say now?

Less rotating mass is always good for any engine!
 
OOPS I forgot to say the important bit!

The out of balance is felt at approx 1500, 3000 and 6000+ rpms

Its got nothing to do with the maximum rpm, its happens several times in the rpm range.

If its worse at 3000, then when you are cruising the bearings will get destroyed...

Around town you go 'up and down' past the problem areas without really noticing it?
 
If those 440 Magnums & Six Packs were poorly balanced from the factory, they sure went well & I do not recall owners complaining. They also seemed to last for tens of thousands of miles.....

As long as the owner matches the piston weights, in my opinion, the engine will be fine for it's intended use. If he did get the assy balanced, I doubt he would notice a before/after difference, except for a lighter wallet....
 
If those 440 Magnums & Six Packs were poorly balanced from the factory, they sure went well & I do not recall owners complaining. They also seemed to last for tens of thousands of miles.....

As long as the owner matches the piston weights, in my opinion, the engine will be fine for it's intended use. If he did get the assy balanced, I doubt he would notice a before/after difference, except for a lighter wallet....
I tend to agree. Many manufacturers engines go 100s of thousands of miles on factory balance. I have road raced many stock engines, revving all thru the range as high as 7500 rpm, and never had an internal engine failure.
 
Just for ref. I did a 69 340 engine that had one rod just over 30 grams out...yeah it ran ok and had over 100k on it when I pulled it apart, but the change after was astounding
 
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